Does the mirror move up/down freely if you push on the mirror?
If the mirror folds out, the motor is good. Any gears or teeth are obviously good- Id check the switch or look for a cold sodder joint. On the trucks there is a calibration/syncronization process where your supposed to fully open/close them 3 or 4 times in a row.
There is a door module that controls all that **** if the motors test out good. These mirrors all operate under the same basic principles as the other GM trucks. I'd bet wiring colors are pretty damn close to the same Try searching for some similar threads like this-
http://www.performancetrucks.net/for...08-gmc-441341/
or some of the other gm truck forums...Might give you some more insight. I think you can fix it.